Electrical connections are an important aspect of many designs. Typical electrical connections include soldering, clamping and lugs. In order to provide reliable long-term connections, good physical contact between the electrical conductors must exist. Soldering accomplishes this by wetting and bonding to the connectors with an electrically conductive material. Clamping and lugs provide a physical force between the conductors to insure intimate contact. If there is not sufficient contact force between the conductors, localized arcing and/or oxidation of the surfaces can occur, resulting in an unreliable connection. For low current static connections, the required contact force to provide a reliable connection is small and can easily be achieved.
For static electrical connections of intermediate and high currents, the required contact force is proportionally higher. (As used herein, “high current” is generally a current at least about 1000 A). Consequently, one must pay closer attention to this contact force because of the potential for arcing to cause physical damage to the connection and render it useless. Typically, these types of connections are bolted together or mechanically clamped and contact surfaces are treated to minimize corrosion.
For high current pulsed electrical connections, the load on the connection resulting from the current is cyclical and the effects of fatigue and creep must be considered. Over time, if not properly maintained, the contact force will diminish and an arc will occur in the connection resulting in permanent damage.
In the field of pulsed power—in which electricity is modulated at high voltage, high current (i.e., high power), and done so over a short time scale—these types of problems are greatly amplified. A short time scale is defined as the regime where thermal, mechanical and magnetic effects do not approach steady state during the discharge (such as, generally less than about 100 ms and, more generally, less than about 10 ms). The forces generated in a connection are large and make fatigue and creep a major problem. For high current electrical contacts, it is generally empirically understood that a minimum of one gram of force be applied per ampere (1 g/A) of current between two surfaces (this is commonly referred to as “Marshall's Law”) or an electric arc will spontaneously form between the surfaces and destroy them. For example, the minimum force required between two surfaces passing a 100,000 A current would be 100 kg or about 220 pounds (lbs.) force. A person of ordinary skill would understand that Marshall's law is a rule of thumb used within the pulsed power industry. If a connection fails due to insufficient contact force, an electric arc will be formed between the two surfaces. The resistance of the arc is generally higher than the contact resistance between the surfaces. Since the energy deposited in a resistor due to current flow is proportional to the square of the current, proportionally more energy is deposited in the interface. If the power deposited in the arc is high enough, the contact material surface can be heated high enough to form a high-pressure plasma between the interface. The high pressure can explosively blow the interface apart, rendering it ineffective as an electrical connection. In addition, its surrounding may be damaged. This process is not too dissimilar to an explosion. For industrial systems, this can result in a loss of equipment, significant equipment down-time and potentially harm personnel.
The reliability of an electrical connection in these environments can be increased by minimizing the contact resistance between the surfaces such as coating the contact surfaces with a highly conductive material such as silver or applying a corrosion inhibitor to the surfaces. Adequate contact force can be made more reliable by using a compliant preload such as one provided by bolts with Belleville washers. These solutions generally work well when the connections are meant to last a long time without servicing. One such integral solution is known by the brand name of Multilam™ (available from Multi-Contact USA of Santa Rosa, Calif.), which minimizes contact resistance between two surfaces by providing multiple, compliant contact points between them. It contains many small louvers made from a spring material that is sandwiched between the surfaces. Each louver acts as a single contact point for each surface. Each louver can act somewhat independently of the others, so it is much more tolerant to surface imperfections, creep and applied clamping force. Since dozens or even hundreds of contact points can be provided in a small contact area, Multilam™ improves contact resistance and reliability over that predicted by a-spot theory which states that no more than three electrical contact points can be guaranteed when two flat surfaces are clamped together. However, because each louver forms essentially a line or point contact, a high contact pressure is imparted and often damages the mating contact surfaces. This problem limits Multilam™ from being used reliably for high current density applications in which the mating surfaces are being moved relative to each other on a repeated basis. (As used herein, the “current density” is current divided by the cross sectional area of the contact; a “high current density” is generally at least about 10,000 A/cm2.)
The above discussion has been centered around static electrical connections. For dynamic connections, in which one surface is moved relative to another while maintaining contact (such as sliding or rotating) one is faced with the additional problem of having adequate preload to prevent arcing between the contacts coupled with the fact that the preload cannot be so high that static friction prevents the surfaces from moving relative to each other. (Such a dynamic connection will also be referred to as a “dynamic contact.”) Furthermore, small imperfections in the surfaces leave them more prone to arcing than nonmovable contact surfaces. This problem is exacerbated when the surface area of the contacts becomes so small that the required preload to prevent arcing nearly deforms the surfaces thereby reducing their lifetime and making them prone to arcing. This problem is also exacerbated when the cross sectional area of the conductor to which it is desired to couple power becomes so small that it becomes difficult to push it through the coupler without buckling it.
In short for dynamic high current applications it is desirable to have the surfaces continually in contact allowing them to slide relative to each other, but have the required clamping force applied to the surfaces only when current is pulsed through them. Extreme care must be taken to make sure that sufficient clamping force is applied every time that the current is pulsed through the contact. One failure may be catastrophic.
All of these connections have one factor in common; they require a high preload force that must be well maintained to prevent catastrophic failure. Because of this, their application in movable electrical contacts in pulsed power applications is limited. Additionally, if the connection sees a current that exceeds its designed clamping force, then the connection will fail.
Thus, there is a need in the art for a mechanism to provide a clamping force in moveable electrical contacts sufficient to prevent catastrophic arcing at the contact while high current is flowing but which permits freedom of relative sliding movement of the contacting conductors when little or no current is flowing. Additionally, there is a further need in the art for a clamping force that adapts to the current carried by the contact.